1. Field of the Invention
The invention relates to an image heating apparatus for heating an image. This image heating apparatus is for use in a copying machine, a printer, a facsimile apparatus or the like adopting an electrophotographic process or an electrostatic recording process.
2. Description of Related Art
Numerous full-color image forming apparatuses such as printers and copying machines using the electrophotographic technique have been put into production.
The performance required of the full-color image forming apparatuses in recent years includes particularly the capability of forming images on various recording materials, a high speed and a high quality of image.
In forming images on various recording materials, it is important for a fixing device to always give an optimum amount of heat to a recording material and a toner. It is because by giving an optimum amount of heat, it is possible to secure sufficient fixing strength and obtain suitable image gloss.
For example, when a thick recording material is used, the thick recording material requires a greater amount of heat than ordinary recording materials to heat and fuse a toner image thereon and fix the toner image, because the thick recording material has a large heat capacity. Accordingly, when the thick recording material is used, a countermeasure is taken by setting a fixing temperature to high level or slowing down a fixing speed to thereby lengthen a fixing time.
However, in the former case where the fixing temperature is set to a high level, if the temperature is made too high, particularly when an image is to be formed on coat paper of low air permeability, there will arise the problem that moisture in the coat paper evaporates at a stroke and becomes vapor, and unevenness forms on a coating layer on the surface of the coat paper and disturbs the image. Also, there are many evils including the accelerated thermal deterioration of a fixing member and members around it and therefore, there has heretofore been adopted the latter countermeasure of slowing down the fixing speed to thereby lengthen the fixing time.
So, there has been proposed an image forming apparatus provided with a plurality of fixing devices so that a fixing process can be well carried out on various recording materials without the fixing speed being lowered (Japanese Patent Application Laid-open No. 2000-221821 and Japanese Patent Application Laid-open No. 2002-214948 (corresponding U.S. Pat. No. 6,512,914)).
However, in such an image forming apparatus provided with a plurality of fixing devices, if the rising operations (preparatory operations)/falling operations (preparatory operations) of the respective fixing devices are performed at the same timing at the start/end of the fixing process, there has been the possibility that it leads to the reduced lives of the fixing devices.